Mass flow metered fuel injection system

ABSTRACT

A mass flow, timed fuel injection system is provided, wherein fuel flow is maintained at all times in a predetermined relation to air mass flow actually entering the engine at any instant, and the fuel injection valve is maintained open for an interval corresponding to a constant crank angle. This interval is corrected to compensate for any error which may have occurred because of inaccuracies introduced by the venturi through which air is metered and to compensate for the variations in discharge nozzle downstream pressure that occur in the induction ports.

United States Patent ONeill 1 Sept. 5, 1972 [54] MASS FLOW METERED FUEL 3,465,732 '9/ 1969 Kattchee ..123/ 139.17 INJECTION SYSTEM 2,957,464 10/1960 Dolza ..123/119 R 2,447,267 8/1948 Mock ..123/1 19 R [72] '"Y Lafayette 3,483,851 12/1969 Reichardt ..123/1 19 R [73] Assignee: Physics International Company, 3,372,680 3/1968 Scholl ..123/13917 San Leandro, Calif. t Primary Examiner-Laurence M. Goodridge [22] filed Apnl 1970 Assistant Examiner-Cort Flint [211 App]. No.: 33,376 Attorney-Lindenberg, Freilich & Wasserman 7 TBA [52] U.S. Cl.....l23/32 EA, l23/l 19 R, 123/140 MC, [5 1 ABS CT 123 139 w A mass flow, timed fuel injection system is provided, 511 1111.131. ..F02b 3/00, F02m 51/06 wherein fuel flow is maintained at all times in a 58 Field of Search ..123/119 R, 139.17, 32 EA, predetermined relation to air mass flow actually emering the engine at any instant, and the fuel injection valve is maintained open for an interval corresponding to a constant crank angle. This interval is corrected to 1403,123/139 AW, 140 MC [56] References Cited compensate for any error WhlCl'l may have occurred UNITED STATES PATENTS because of inaccuracies introduced by the venturi through which air is metered and to compensate for Gulot the variations in d sc a ge o le downstream p e 2,899,948 8/ 1959 Groves 123/ 140.3 sure that occur in the induction porw 3,240,191 Wallis ..123/32 EA DU RATlON CONTROL 15 Claims, 7 Drawing Figures ENG\N E mrrine 1 MANIFOLD l i MOTOR ALTERNATOR MASS FLOW METERED FUEL INJECTION SYSTEM BACKGROUND OF THE INVENTION This invention relates to fuel injection systems for intemal combustion systems and more particularly to improvements therein.

In an application for patent entitled Fuel Injection and Control System, filed Sept. 17, 1969, Ser. No. 858,717, now abandoned, by this inventor, which is assigned to a common assignee, there is described a common rail fuel delivery system for an internal combustion engine wherein the pressure at which fuel is applied to the fuel injection valves in the system, is in proportion to the mass air flow, information for which is derived using a venturi. The duration of each of the fuel injection valve open times is varied inversely with engine speed. To avoid the formation of vapor at low engine output, together with inadequate atomization, fuel pressure is prevented from falling below a predetermined minimum value which is preselected. At all operating power levels below this minimum value fuel pressure, it is apparent that mixture strength would be too rich. To maintain correct mixture strength under these conditions, a correction in fuel delivery valve open duration time is introduced that shortens the duration of injection in the relationship:

a v a where T is actual injection valve open duration, T, is injection valve open duration derived from fixed crank angle, P, is a fuel injection pressure derived from mass air flow signal, and P, is actual fuel injection pressure. For these purposes, the actual fuel injection pressure used is taken as the pressure differential between th fuel supply line and intake manifold pressure.

To avoid the problems produced by using constant time per injection, the open time of each of the injection valves per cycle is in inverse proportion with speed, or, the injection valve is open for a constant crank angle when fuel injection pressure exceeds some predetermined level, such as psi.)

The system described briefly, employs a venturi depression signal to modulate the common rail feed fuel pressure. While the system operates satisfactorily, improvement is noted when certain factors which occur under specific operating conditions of the venturi are taken into consideration. One of these is the fact that velocity and pressure pulsations that alter in phase can cause the venturi meter to produce a false measurement of air flow. To avoid this problem efforts are made to attenuate pulsation at the venturi. The venturi is positioned upstream of an air surge damping capacity, remote from the engine ports and, since no fuel is required to be fed through it, problems of fuel transport and deposition are avoided.

A second error occurs in mass air flow measurements made by venturi meters at high throat vacuum, when the expansion of the air mass creates a depression signal that does not follow a squared law. The error increases in magnitude as a result of variations in ambient temperature and ambient pressure. Accordingly, in order to avoid the error in air mass flow measurement by a venturi meter, which is caused by these factors, some arrangement must be found to measure these error causing factors and make the proper corrections for them.

An object of this invention is to provide in a fuel injection system of the type wherein fuel injection pressure is determined by air mass flow as measured by a venturi meter, an arrangement for compensating for certain inaccuracies introduced in the measurement of said air mass flow by said venturi meter.

Yet another object of this invention is the provision of an air mass flow timed fuel injection system which compensates forerrors caused in air mass flow measurement by a venturi meterby ambient temperature and pressure variations and to provide accurate fuel mixture control at all altitudes.

Still another object of the present invention is the provision of a novel and improved fuel injection system.

These and other objects of the invention may be achieved in a common rail fuel delivery system of the 0 type described briefly above, wherein an analog electrical circuit has applied thereto signals from several sensors which respectively measure the ambient temperature, the fuel temperature, atmospheric pressure, the pressure drop between the fuel supply pressure line and the manifold, the depression in the venturi throat below atmospheric pressure, and the depression below atmospheric pressure in the idling control channel which is down stream of the venturi, and from all these signals, the analog circuit produces a resultant analog electrical signal which is used to shorten the duration of the constant crank angle open time of the fuel injection valve, whereby any error in the measurement of the air mass flow is compensated.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description, when read in conjunction with the accompany drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic arrangement illustrative of where the various sensors required with this invention are placed.

FIG. 2 is a schematic diagram of an analog computer used to calculate the venturi meter error signal in accordance with this invention.

FIG. 3 is a schematic circuit diagram of a fuel injection valve control circuit.

FIG. 4 is a pulser circuit diagram which is employed with this invention.

FIG. 5 is a discharge circuit diagram which is employed with this invention.

FIG. 6 is a circuit diagram of a starting inverter which is employed with this invention, and

FIG. 7 is a schematic diagram of another embodiment of the fuel injection valve control circuit which may be employed in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS There follows a derivation resulting in an equation which is implemented by analog circuits to provide the reduction in the open time of fuel injection valves which compensates for any errors introduced into the mass air flow fuel metering system as a result of the use of the venturi for determining mass air flow. From the equation which expresses fuel flow in pounds per G: In xv 1) Pl; 2 second in an interrupted flow system, and from an v 1"? 27- 7X) equation which approximates air flow in a continuous mfiw. m. flow system through a venturi, there is derived a cor- \/Q rection factor which can be used to correct the crank P1 angle signal which represents the interval over which the fuel injection valve 'is to be maintained open, with this approximation G, one finally has whereby compensation is provided for the error in the measurement of mass air flow through the venturi me- W: 1 1 i a} ter. 10 V'y1 gRT First there will be shown the derivation of an approx- 2y 9 P Pn PD) .2? O.3()( imation for air flow through a venturi which is then em- W CdvAv v 1 h Tl P, 0 P,

ployed with the equation expressing fuel flow through a venturi, for obtaining the crank angle correction factor. This approximate formula for W is y a o in Ai fl through a venturi f ll the law the ranges of P and P, that is under consideration. The

table on the next page shows the accuracy of the ap- Accuracy of the Approximation to W for 7 =1.4

where cdv Coefficient of discharge of venturi Range of Maximum Relative Error in w W flow rate l Exact-Approximation/Exact X 100% a throat area (in?) I g units conversion factor 3 g 3-2 k w g:: P:/% i 5- P ambient pressure T ambient temperature P throat pressure R gas constant 7 gamma law gas factor Assume now that Adiabatic conditions 7= 1.4. The Fuel flow lb/sec (interrupted flow) for an interval combustion engine may be expressed as In order to build a simple electrical analog computer Lf= A V p; tN A Q, 2gh,p, tN to compute w, it is necessary to approximate this function using only the operations of addition, subtraction, h L! A" t X N multiplication, division, and the evaluation of a square w ere mot A Nozzle orifice area Q,-= Nozzle coefficient of discharge p,= Fuel density h,= Head loss across nozzle The quantity in parenthesis involves only these 5 operations, so it is now necessary to find an approximation to W P,= Pressure drop across nozzle psi) G: m t Duration of in ection (sec) 1 1 N Number of injections/sec This can also be expressed as 40 A f venturi throat i /W P Ambient pressure psi) G: N (15;) 1 T Ambient temperature R) Let P P P P Venturi throat depression psi) so an; 2/2,] 0/ l) C =Coefiicient of discharge of venturi It was previously shown that an approximation of Air thls substlmnon G becomes flow, lb/sec continuous flow) through a venturi may P PD) 1 be approximately expressed as I: "T *T 171 11 Pl g 1 PD I PD 1 iv d'n i 1'1? 7*, F For this particular application which 18 carburetion 'Y 1 1 I 1 (2) system, the only valves of h and P of interest are in the Ifw =3 Lfwhere B =air/fuel ratio, then equating B ranges X Equation (1) with Equation (2) 0 3 P 3 oinchesofl-lg l5 3 P 3 3linchesofl-Ig so that the range of the dimensionless ratio P /P is B ANCdNiN/% /Ff= 0 S o/P1) s (6/15)= 0.4 The smallness of the ratio P lP suggests that Cdva ll, (E 028541306 E v1 R T, W P, P, fi 5 P1 P 0.2860.306 IT 1 1 1 D be expanded in a Taylor series about P lPl 0 to get an K13 V F W approximate expression for G. Thus, using when,

m 1 52 i i E) K Ce li l) :1 1r i) v "r 1 X a 7 of and neglecting all terms higher than second order in K 0.286I,0.306P 5 1 PO/Pay, gives 1 T I, t-N (3) where 1 Nt\/ m a constant K Therefore substituting for t in Equation 4) 0.245 psi Without correction Where a=injection duration in crank angle degrees.

a 440 60 r.p.m. (4.275)

519 0.286P,0.306PD 60 r.p.m. T,

4 t Tl r.p.rn. where In Equation (7) the correction factor is:

This'additional term gives a total correction factor that can be applied to valve opening duration:

Correction factor M l or PD Correction factor=K 440 W fflaai itryaeyisi Ti where K 440 w/4.275 (8) The correction factor is P1) 7 1/2 K5 mason--0.500% (8A) PFA difference between fuel pressure and manifold pressure is obtained electrically from three sensors and an analog circuit. An electrical signal proportional to 'the full 01 injection event is obtained from a ramp generator. This signal is reduced by multiplying it by the correction factor and the resultant signal is employed to terminate injection at a crank angle duration correspondingly less than a".

Reference is now made to FIG. 1 of the drawings. This FIG. 1 resembles FIG. 1 of application Ser. No. 858,7l7 referred to above, except modifications required for the purpose of this invention. Thus, three additional sensors are provided, one of which is a barometric pressure sensor, the second an ambient pressure sensor, and thethird a fuel temperature sensor. This drawing also shows the presence of other structures which are used in accordance with this invention.

A high pressure pump 10 is driven in approximately constant relationship to engine speed, either by direct coupling, or preferably, by an induction motor 12 which is powered directly from the AC output of the vehicle alternator 14. The pump 10, which may be a positive displacement pump, pumps fuel from a conventional float bowl 17. A float .16 enables fuel to be transferred into the float bowl 17 from the fuel tank, when the level of the fuel in the float bowl drops below a predetermined value.

The fuel is pumped to a plurality of injection valves 18A, 18N, one to each cylinder, which are fed from a common passage. The fuel is then circulated to a spill valve unit 20, and from the spill valve unit back to the float bowl 17 then to the injection valves. Each ofv the injection valves may be of an electro mechanical type that open and close very rapidly in response to an electrical signal, which is provided by open duration control system 22. The in'ection valve may also be of a hydraulically actuated igpe which responds to hydraulic pressures from the" opening duration control 22. Each valve contains a calibrated orifice through which the fuel must pass before it can be discharged.

The spill valve unit 20 includes a cylindrical chamber 23 having a diaphragm 24 mounted therein. The diaphragm is attached to a poppet valve stem 26 which is supported in a guide bore 28. The lower part of the guide bore 28A is enlarged and the larger bore terminates in a narrow seating upon which the poppet valve 30 can seat. The poppet valve is free to slide on its stem but is urged toward the closed or seated position by a spring 32 mounted on the stem between the poppet and a snap ring or collar 34. To prevent the spring from drawing the stem downward and thus distorting the diaphragm, a collar 26A is formed at the upper end of the valve stem that abuts the spill valve housing when the diaphragm is in the lowest permitted position.

The air silencer and cleaner 36 is supported on one end of the air intake 38 which leads to a common plenum chamber or manifold (not shown) which distributes air to the intake ports of the engine. A venturi structure 40 is located in this intake. A throttle valve 42 is located between the venturi and the engine. Tappings in the throat of the venturi, .44, are connected via a tube 46, to the diaphragm chamber 23 of the spill valve unit 20, to apply vacuum to the upper side of the diaphragm 24, depending upon the flow of air through the venturi. The greater the flow of air, the greater the pull on the poppet valve, the less fuel can get by it and therefore the higher the back pressure of the fuel on the injection valves. The lower the flow of air through the venturi, the lower the pull on the diaphragm and the poppet valve, the more fuel can get by the poppet valve into the float chamber, and therefore the lower the pressure of the fuel on the injection valves.

A progression system and idling port 50, with a volume control tapered screw 52, and an air bleed from atmosphere 53, which are conventionally used in carburetors, are provided in the throttle body and the channel linking, idling and progression holes is connected to a third or idling pressure sensor 54, which will be described later herein. An air bleed line 56 is used to vent the idling port 50. a

A mode switch 58, which will be explained in more detail in FIG. 2, responds to an indication from the throttle valve that an optimum output is required. That is, when the fuel mixture needs to be altered from say, a cruising air fuel ratio of 16:1 to a full power ratio of 12:1. The mode switch then signals the control circuits 22 to lengthen the fuel injection interval.

A first pressure sensor 60, comprises an upper chamber 60U which is separated from the lower chamber 60L by a diaphragm 60D. The diaphragm carries a magnetic core 62. A winding 66 senses the position of the core 62. At this point, it is desired to emphasize that the foregoing pressure sensor construction is merely exemplary not only of pressure sensor 60, but others to be described herein. Other well known pressure sensing transducers may be employed, such as the strain gauge type, without departing from the spirit and scope of this invention.

The upper chamber of the first pressure sensor 60 is connected to the upper side of the spill valve diaphragm chamber. The lower chamber of the first pressure sensor is connected via a pipe 68 to the lower side of the spill valve diaphragm chamber. The lower side of the spill valve diaphragm chamber also communicates via the pipe 70 with the float bowl. The first pressure sensor thus provides an electrical output, designated as Pdv and applied to control 22, which is representative of the difference in pressure across the diaphragm which is proportional to the intake air (mass flow) or to the fuel pressure that is being signalled to the injection valves. A tube 41 also extends from the air intake 38 to the tube 68 and then to the tube 70. An impact tube 43 is placed at the junction between tube 41 A second pressure sensor 72 has a similar construction to the first pressure sensor. lts lower chamber 72L is connected via a tube 73 to the intake manifold downstream of the. throttle valve 42. The upper chamber 72U is connected via a pipe 74 to the fuel passage in the spill valve unit 20 which is upstream of the poppet valve 30. An inductive pickup 76 on the second pressure sensor 72 senses the position of the diaphragm therein.

The electrical signal output of the second pressure sensor, designated as Pfa, is applied to the control 22 and represents the actual fuel pressure difference applied across the calibration in the injection valves which is to be described later.

The idling pressure sensor 54 may be called the third pressure sensor and is connected so that the idling and progression channel is in communication with the upper side of the diaphragm 90 which is carried by the third pressure sensor, while the lower side of the diaphragm is vented to the air intake horn between the air silencer and the venturi by being connected to tube 41. An electrical pickup 94senses the displacement of the diaphragm. The electrical signal output of the third pressure sensor designated as PDT, is applied to control 22 and indicates (air mass flow) over periods of light engine operation when the flow through the venturi 40 falls into a laminar flow regime and indicates flow erroneously.

The third pressure sensor output is used to indicate when the fuel pressure would fall below a predetermined value, for example below 20 lbs. per sq. in. lf the determination of fuel pressure remained solely under the control of mass air flow in the venturi, at low engine output there would be a low fuel pressure and vapor would be formed and there would be inadequate atomization. The spring 32 establishes a minimum fuel pressure which avoids the effects of low fuel pressure. However, at engine operating levels below this minimum value fuel pressure the mixture strength could become too rich. A correction is made bythe open duration control 22, in the time duration the fuel delivery valve is open, that shortens fuel injection time and thereby compensates and maintains the correct fuel mixture strength. The barometric pressure sensor 96 and the ambient temperature sensor 98, are both placed in the air cleaner. In this way the barometric pressure sensor takes into account the additional pressure loss due to filter element resistance. The output of the barometric pressure sensor is an electrical signal designated as P representative of the barometric pressure. The ambient temperature sensor produces as its output an electrical signal, designated as T,, representative of the ambient temperature. There is also a fuel temperature sensor 100, which is placed in the bowl 17 and which produces as its output a signal T, representative of fuel temperature. All of the foregoing signals are applied to the Open Duration Control Circuits 22.

When the engine is first started, the choke valve 39 placed just before the venturi is turned to block the passage of air. This causes an increase in the differential air pressure across the diaphragm 24. The result is an upward pull on the diaphragm which pulls upon on the spill valve. This results in increased pressure and fuel flow to the fuel injection valves, a desirable condition during cold starting.

Reference is now made to FIG. 2 which is a block schematic diagram of an analog circuit which implements equation 8A. This is the equation for the correction factor by which it is required to reduce the constant crank angle fuel injection interval.

The output of the pressure sensor 60, which senses a venturi pressure, and which is designated as P,,,. is applied to an amplifier 102. The output of this amplifier, which is non-inverting, is applied to a diode 104. The output of the diode is applied to an attenuating potentiometer 106, which multiplies the value P,,,. by 0.306. Thereafter, the attenuator output is connected to a summing amplifier 108 input.

The output of the ambient temperature sensor 96, which is designated as P is applied to an inverting amplifier 110. Its output, represented by P is applied to an attenuating potentiometer 112, which attenuates it by the factor 0.288. The output of the potentiometer 112 is applied to the summing amplifier 108 as its second input.

The output of the summing amplifier, which now comprises (0.286P +0.306 P,,,.) is applied to an amplifier 114, which multiplies it by a gain factor K The output of the amplifier 114 is applied as one input to a multiplier circuit 1 16.

The output from pressure sensor 54, which derives a signal from the idling control channel, and which is represented by P is applied to a noninverting amplifier 118. The output of this amplifier is applied to a diode 120. The output from diodes 104 and 120 are connected together and serve as one input to a multiplier circuit 122. The signal that is applied to the multiplier 122 will be the larger of the two signals applied to the diodes 104 and 120, since this larger signal will bias off the other diode. Thus, the signal applied to the multiplier 122 represents either the idling air pressure or the off idling air pressure.

The other input to the multiplier 122 is a correction factor which is derived from the fuel temperature sensor 100, and is represented by the signal 7}. This is applied to an inverting amplifier 124. The output of this amplifier is applied as a second input to the multiplier 122.

The fuel pressure sensor 72, represented by the signal Pm iS applied to an inverting amplifier 126. The output of this amplifier is applied at one input to a multiplier 128. The other input to the multiplier is the ambient temperature sensor 98 output, T which is ap plied to an inverting amplifier 130. The output of amplifier 130 is the second input to multiplier 128.

The reason for the correction for fuel temperature provided by the signal T,, is that the variation in fuel gravity is approximated by a straight line relationship of 7 percent mass flow reduction for 100 F. fuel temperature increase. The purpose of the fuel temperature sensor 100 is to produce a signal which increases injection duration with rise in temperature to compensate for this effect.

The output of the multiplier 128, consisting of the quantity P f is one input to a multiplier 132. This multiplier 132 is in the negative feedback path of the amplifier 134. This negative feedback is applied to the multiplier 132 as its second input. The output of the multiplier 132 is summed with the output of multiplier 122 and applied to the input of amplifier 134 As a result, the output of amplifier l34representsthe output of multiplier 122 divided by the output of multiplier or (l! f)/( fu. l)-

The output of amplifier 134s applied to the multiplier 116 as its second input. The output of the multiplier 1 16, consisting of the product of its two input terms, is applied to an inverting amplifier 136. This amplifier has its output connected as two inputs to a multiplier 138, whereby the output of the amplifier constitutes the square root of its input.

A switch 140, responsive to movement of the throttle control pedal, is fitted so that the output of amplifier 136 may be directly passed to a multiplier 162 when the switch is closed (throttle full open) or may be reduced by the voltage dividing action of a potentiometer 139 when the switch is open (throttle partially closed). The action of the switch, when it is open, is to reduce the voltage applied to multiplier 162 and thus to reduce fuel supply by shortening the injection interval when the throttle is less than fully open.

The setting of the potentiometer 139 is done at the factory. It is determined by the fuel/air ratio needs of a particular engine, i.e., at normally cruising ranges this potentiometer enables the fuel/air mixture to be made as lean as can be tolerated and yet still be under control of the analog computer. When large engine power or a high speed is needed, the switch 140 is operated. The signal applied to the multiplier 142 is an unattenuated output of the analog computer circuit, which is of course larger than the attenuated signal. This, as will be seen subsequently herein, results in the interval over which the fuel injection valves are maintained open, being greater and therefore the fuel mixture which is injected being richer, whereby the requirement for increased engine power is met. However, the valve open interval is still modified by the correction signal computed by the analog computer described.

FIG. 3 is a block schematic diagram of an arrangement for generating an electrical signal representing a fixed crank angle interval, which is multiplied with the correction signal to produce a resultant signal for correcting fuel injection valve open interval at the particular speed of operation. In essence this constitutes a correction in the fuel delivery valve open interval which shortens the duration of injection in the relationship T T (correction factor), where T is the actual injection valve open duration, and T is injection valve open duration derived from a fixed crank angle. Two shafts respectively 14, 144 are driven by the engine crankshaft at engine speed. Shaft 142 carries a single contact 146 which, in the course of rotation successively touches contacts 148 and 150. When the connection between contacts 146 and 148 is made, a pulse generator 152 is enabled to generate a pulse which is applied to a ramp generator 154 causing it to commence to generate a ramp. When contact 146 connects with contact 150, a second pulse generator 156 is enabled to apply a pulse to a time delay circuit 158, which after a predetermined interval resets the ramp generator 154. This interval is determined by the time required for the output of pulse generator 156 to enable a sample and hold circuit 160 to sample the output of the ramp generator at that time. As a result, sample and hold circuit 160 has an electrical signal whose amplitude represents the fixed crank angle interval between the contacts 148 and 150 at the particular speed of rotation.

The output of the sample and hold circuit 160 is applied as one input to a multiplier 162, whose other input is the correction factor signal derived from the output of the amplifier 136 in FIG. 2.

The shaft 144 has four contacts respectively 144A, B, C, and D positioned at four quadrants. These will .successively connect with a contact 162 as the shaft rotates, and also with a contact 164. When the contacts on shaft 144 connect with contact 162, they cause a pulse generator 166 to apply a pulse to a ramp generator 168 enabling it to commence to generate a ramp voltage signal, and is also applied to a pulse circuit 170, enabling it to produce an output in response thereto.

Similarly, contact 164, when connected to the successive contacts 144A through D, enables the pulser generator 172 to generate a pulse whereby ramp generator 174 is enabled to commence generating a ramp voltage and pulser 170 is caused to generate an output pulse.

The output of the ramp generator 168 is applied to a voltage comparator 180, which functions to compare the ramp voltage with the output of the multiplier 162. When the ramp voltage equals the multiplier voltage, then the voltage comparator 180 provides an output signal which resets the ramp generator 168 and which enables a discharge circuit 182 to discharge a voltage which may be across any one of the respective injection valves respectively 190, 192, 194, or 196. The discharge circuit thereby closes any one of these valves which may have been open. I The discharge circuit discharges into the power supply, 184, for the system thereby conserving energy.

The output of the ramp generator 174 is applied to a voltage comparator 186, whose other input is the output of the multiplier 162. When the ramp generator voltage equals the multiplier voltage, then the voltage comparator 186 produces an output which resets the ramp generator and which enables a second discharge circuit 188 to discharge any voltages into the power supply 184, which may be across any one of the injection valves 191, 193, 195, or 197. Thereby, which ever one of the valves was open at this time, is discharged. The operation of the system described in FIG. 3, is as follows: The contact 150 is spaced at a location relative to contact 148, which represents the maximum number of crankshaft degrees over which injection is required to occur. The ramp generator 154 is energized when contact 146 connects with contact 148 and generates a ramp voltage over the interval required for contact 146 to travel the indicated crankshaft angle until it connects with contact 150. At this time the sample and hold circuit 160 is energized and the ramp generator 154 is reset. The sample and hold signal then holds a signal whose amplitude represents the time taken for the shaft to rotate the predetermined crankshaft angle. From a time standpoint, this is also representative of the maximum time that the fuel injection valves should remain open. This voltage signal is renewed for every rotation of the shaft 142.

The output of the sample and hold circuit 160 is reduced by an amount determined by the correction factor signal generated by the analog computer shown in FIG. 2. This signal is a voltage amplitude less than I.

As soon as it'attains the value I, then the interval that the fuel injection valve remains open is determined solely by the time required for the engine to rotate through the predetermined crank angle.

Ramp generator 168 is energized at four successive times during the rotation of the shaft 144. Ramp generator 174 is also energized at four successive times during the rotation of the shaft 144. Each time each one of these ramp generators is energized, pulser is energized. Accordingly, pulser 170 delivers eight successive output pulses to a distributor 189. The distributor successively applies these output pulses to the respective fuel injection valves through 197.

A suitable fuel injection valve, which may be electrically controlled by the output of the pulser circuit, may be valves of the type described for example, in a patent by N. Kattchee (application Ser. No. 676,458), U.S. Pat. No. 3,465,732 or a patent by G. Benson (application Ser. No. 671,065), U.S. Pat. No. 3,501,099. The nature of the piezoelectric material used in the fuel injection valves is that upon the application of a voltage pulse thereto, it moves and thereby enables fuel to be injected into an intake post. The piezoelectric material will remain in its moved condition, acting as a capacitor storing the electrical energy supplied thereto. The energization of the discharge circuits 182 or 188 removes the voltage from across the piezoelectric material enabling the piezoelectric fuel injection valve to be closed.

The purpose of the respective diodes 100 through 107 which are connected between the respective piezoelectric valves 190 and 197 and the respective discharge circuits 182 and 188, is to prevent a voltage being applied to one valve such as 190 to cause it to open, from also being applied to the other three valves, respectively 192, 194 and 196, with which it makes a common connection.

The power supply 184 includes an AC alternator and a regulator normally used with an internal combustion engine, and in addition contains a storage capacitor at its output, into which the discharge circuit voltages may be dumped in order to conserve energy. A starting inverter circuit 110, which is shown in FIG. 6, is energized by the battery 112, energizes the pulser circuit until the engine starts whereby its alternator can supply the required energy.

FIG. 4 is a circuit diagram of the pulser circuit 170, which may be employed with this invention. A capacitor 214 is charged up from the power supply 184. The capacitor is connected in series with two matched silicon controlled rectifiers respectively 216 and 218. The control electrode of silicon control rectifier 218 is connected to the secondary winding 220 of a transformer, having a split primary winding respectively 222, 224. The cathode of the silicon control rectifier 218 is also connected to an inductor 226 whose other end is connected to the distributor 189. The ends of the respective primary winding 222 and 224 are respectively connected to 'the pulse generators 166 and 172.

In the presence of a pulse from either pulse generator 166 or 172, the silicon control rectifier 218 is enabled to conduct. The voltage across the storage capacitor 214 atthis time is sufficient to break down the silicon controlled rectifier 216 which then acts as a diode. A voltage pulse is then applied to the distributor 189.

The discharge circuit is shown in FIG. 5. Two of these are used. The anode ends of four of the diodes 100 through 107 are connected to two series connected matched silicon control rectifiers, respectively 230, 232. The control electrode of the silicon control rectifier 232 is connected to a secondary winding 234 of a transformer. The primary winding of the transformer 236 is connected to the output of one of the voltage comparators 180 or 186. The cathode of silicon control rectifier 232 is connected in series with a damping inductor 238. The other end of the damping inductor is connected to the primary winding 240 of a transformer. A free wheeling diode 242 is connected between the first end of the inductor and ground. The secondary winding 244 of the transformer is connected through a locking diode 246 to the storage capacitor 214.

When a voltage comparator produces an output, it enables silicon control rectifier 232 to conduct whereby the voltage which was applied across one of the fuel injection valves is enabled to be discharged into the primary winding of the transformer 240, inducing a voltage in the secondary winding 244 which is dumped into the storage capacitor 214. The purpose of the diode 242 is to prevent a reverse current flow through transformer primary winding 240. The diode 246 prevents a negative voltage swing at the fuel injection valve.

FIG. 6 is a circuit diagram illustrative of a starting inverter circuit, which may be employed with the embodiment of the invention. This includes effectively an oscillator circuit 250, which is energized when a start signal, is applied thereto. The start signal may be initiated from the battery when the ignition key is turned. The oscillator oscillates and builds up a charge across a capacitor 252, which is applied to the pulser circuit 170 enabling it to function for starting the car engine. Once the engine has fired, a stop signal is applied to the oscillator to discontinue its operation.

FIG. 7 illustrates an embodiment of the invention which may be employed with electrical solenoid fuel injection type valves instead of piezoelectric type valves. In this arrangement, the rotating shafts and the contact arrangements associated therewith, the ramp generators and sample and hold circuits are the same as was described in connection with FIG. 3 and therefore similar functioning structure will bear the same reference numerals. However, since substantial current must be applied to the solenoid type fuel injection valves, it is necessary to employ some type of a switching device to maintain battery current applied to these valves. Accordingly, the output of pulse generator 166 turns on a solenoid switch 260 whereby current can be applied from a battery or power supply 262, to a distributor 264. The distributor distributes current successively to four fuel injection valves respectively 265, 266, 267 and 268. When the output of the voltage comparator I80 occurs, it resets ramp generator 168 and turns off solenoid switch 260 thereby discontinuing current from the battery 262.

In the presence of an output from pulse generator 172, solenoid switch 270 is energized enabling current to flow from the battery 262 to a distributor 272. This distributor controls the application of current to each of the four valves respectively 273, 274, 275 and 276. In the presence of an output from voltage comparator 186, the solenoid switch is rendered inoperative thereby preventing further current flow from the battery 262 to the distributor 272.

It should be appreciated that the correction signal generated by circuit arrangement of FIG. 2 may also be employed with a hydraulic fuel injection valve control system as described in FIGS. 3 through 10 of the application for patent'Ser. No. 858,717, by this inventor which has been mentioned previously.

There has accordingly been shown and described above a fuel injection system wherein fuel injection valve opening time occupies a constant crank angle at all engine speeds and the common rail feed pressure is modulated in proportion to a venturi depression signal, with provisions further being made for correction of the valve opening interval for errors introduced by the venturi. During those periods when the operation of the engine is at light loads fuel pressure is fixed. Corrections are applied to the valve opening interval during these periods to prevent too rich a fuel mixture from occurring. The corrections applied are generated by an analog computer which may be termed a duration control computer. This computer responds to the error in fuel flow produced by the spill valve system when responding to a signal from the venturi throat. It also senses the error in fuel flow produced bythe actual pressure drop across the fuel nozzle compared with the pressure drop across the spill valve.

The first of these errors results from air compressibility. As venturi depression increases, ambient pressure reduces or ambient temperature increases, the relationship between air flow and throat depression undergoes a change from a squared law to a higher index. The computer senses these parameters and determines a correction factor. 5

The second error rises when either (a) manifold pressure falls significantly below atmospheric and/or (b) the spill valve collar contacts the housing upsetting the force balance between fuel pressure and venturi depression. The computer senses fuel pressure drop across the discharge nozzle and in combination with venturi depression it produces a second correction factor in proportion to the pressure error existing.

These corrections are multiplied together and the resultant correction is employed to adjust the duration of injection.

What is claimed is:

1. A system for controlling the amount of fuel introduced into the combustion chambers or ports of an internal combustion engine by fuel injection valves fed from a common rail, each fuel injection valve being opened for fuel injection over an interval required for said engine to rotate through a fixed crank angle, said engine having an air intake with air flowing through a venturi meter, the improvement comprising:

valve means for maintaining fuel pressure applied to said common rail from falling below a predetermined value despite engine output falling below a level required for maintaining said predetermined value,

means for controlling said valve means for varying fuel pressure applied to said common rail above said predetermined pressure value as a function of the air mass flow to said engine flowing through said venturi meter,

electrical analog means coupled to said internal combustion engine for generating a correction signal representative of errors introduced into the air mass flow function due to air compressibility and for errors occurring when said fuel pressure is maintained at said constant value, and means responsive to said correction signal to modify said crank angle fuel injection interval over which said injection valves are held open to compensate for said errors. 2. A system as recited inclaim 1 wherein there is included means for establishing a crank angle signal representative of the interval over which said fuel injec-- tion valves should remain open,

I said means for modifying the fixed crank angle interval over which said fuel injection valves are open includes means formultiplying said crank angle signal with the output of said analog means to produce a resultant signal representative of the duration for'which said fuel injection valves should remain open.

3. A system as recited in claim 1 wherein there is included an idling control channel which follows said venturi meter, and said air flowing through said venturi meter thereafter flows through said idling control channel, and said means coupled to said internal combustion engine for generating a correction signal includes means for measuring air pressure at said venturi and producing a first signal representative thereof,

means for measuring air pressure in said idling control channel and generating a second signal representative thereof,

means for measuring ambient temperature and providing a third signal representative thereof, means for sensing barometric pressure and producing a fourth signal representative thereof,

means for determining which of said first and second signals is the larger,

means for modifying said larger signal with said third and fourth signals to provide a modified signal representative of the substantially correct air mass flow through said venturi.

4. A system as recited in claim 3 wherein there is included a means for measuring fuel pressure applied to 6. In a system for controlling the amount of fuel introduced into the combustion chambers or ports of an internal combustion engine by fuel injection valves fed from a common rail, each of said fuel injection valves being opened for fuel injection over an interval determined by the time required for said engine to traverse a predetermined crank angle, said engine having an air intake through a venturi meter followed by an idlying said fuel intake manifold and establishing a'fuel pres- I sure signal representative thereof,

said valve means is a spill valve having a predetermined valve area,

a diaphragm is coupled to said venturi meter and to said spill valve to move said spill valve in accordance with the air pressure variations at said venturi meter,

means for increasing the amplitude of said modified signal by a constant determined by the ratio of the area of said diaphragm to the area of said spill valve to produce an amplified modified signal,

means for determining the ratio of said fuel pressure signal and said amplified modified signal to produce a sixth signal representative thereof,

means for determining the square root of said sixth signal and producing a correction signal representative thereof.

5. In a system as recited in claim 1 wherein there is included mode switch means for altering the amplitude of said correction signal responsive to predetermined load requirements on said internal combustion engine.

control channel, the improvement comprising:

valve means for maintaining fuel pressure applied to said common rail from falling below a predetermined value despite engine output falling below the level required to maintain said value, means for controlling said valve means for varying fuel pressure of fuel applied to said common rail above said predetermined pressure value as a function of the mass air flow to said engine flowing through a venturi meter and thereafter through an idling control channel, means for varying said crank angle interval over which said injection valves are held open to compensate for errors introduced into the air mass flow function because of air compressibility and for errors occurring when said fuel pressure is maintained at said predetermined value, including means for measuring air pressure at said venturi and generating a first signal representative thereof,

means for measuring air pressure in the idling control channel and generating a second signal representative thereof,

means for sensing fuel injection pressure to said common rail and providing a third signal representative thereof, means for measuring ambient temperature and providing a fourth signal representative thereof, means for sensing the barometric pressure and providing a fifth signal representative thereof,

means for sensing the fuel temperature and producing a sixth signal representative thereof, and

electrical analog means responsive to said first, second, third, fourth, fifth and sixth signals for generating a correction signal representative of an amount said fixed crank angle interval should be altered to compensate for errors introduced in the mass air flow function because of air compressibility and for errors occurring when said fuel pressure is maintained at said predetermined value caused by low engine output, and

means for reducing the fixed crank angle interval over which said fuel injection valves are open responsive to said electrical analog means output.

7. In a system as recited in claim 6 wherein there is included means for establishing a crank angle signal representative of the interval over which said fuel injection valves should remain open,

said means for reducing the fixed crank angle interval over which said fuel injection valves are open includes means for multiplying said crank angle signal with the output of said analog means to produce a resultant signal representative of the duration for which said fuel injection valves should remain open.

8. In a system as recited in claim 6 wherein there is included mode switch means for altering the amplitude of said electrical analog means output responsive to predetermined load requirements on said internal combustion engine.

9. In a system as recited in claim 6 wherein said electrical analog-means comprises means for establishing which ever of said first and second signals is the larger and producing a seventh signal representative thereof,

means for combining saidfifth and seventh signals for producing an eighth signal representative thereof,

means for increasing said eighth signal by a predetermined gain to produce an eighth signal representative thereof,

means for multiplying said seventh and sixth signals to produce a ninth signal representative thereof,

means for multiplying said third and fourth signals to produce a 10th signal representative thereof,

means for dividing said ninth signal by said tenth signal to produce an 1 1th signal representative thereof,

means for multiplying said eighth signal and said 1 1th signal to produce a 12th signal representative thereof, and

means for taking the square root of said 12th signal to produce a 13th signal representative of a cor-- rection factor for correcting the interval during which the fuel injection valve should be held open. 10. In a system for controlling the amount of fuel introduced into the combustion chambers or ports of an internal combustion engine by fuel injection valves fed from a fuel intake manifold, each fuel injection valve being opened for fuel injection over a crank angle interval required for the engine to rotate through a predetermined crank angle, and the pressure above a predetermined threshold at which said fuel is applied to each said injection valve is varied as a function of the air mass flow to said engine through a venturi meter and thereafter through an idling control channel, means for correcting the interval over which said fuel injection valve remains open to compensate for errors in fuel pressure which occur due to errors introduced by the use of a venturi meter for measuring air mass flow to said engine, and to control the richness of the fuel mixture when said fuel pressure attains its predetermined threshold value comprising:

first means for sensing the pressure of air at said venturi throat and establishing a first signal representative thereof, second means for sensing the air pressure :in the idling control channel which is downstream of said venturi throat and producing a second signal representative thereof, third means for sensing fuel pressure applied to a fuel intake manifold and producing a third signal representative thereof, fourth means for sensing ambient temperature and producing an electrical signal representative thereof, fifth means for sensing the barometric pressure and producing a fifth signal representative thereof, sixth means for sensing the fuel temperature and producing a sixth signal representative thereof, means for establishing whichever is the larger of said first and second signals and producinga seventh signal representative thereof, means for combining said fifth and seventh signals for producing an eighth signal representative thereof,

means for increasing said eighth signal by a predetermined gain to produce an enlarged eighth signal representative thereof,

means for multiplying said seventh and sixth signals to produce a ninth signal representative thereof, means for multiplying said third and fourth signals to produce a 10th signal representative thereof, means for dividing said ninth signal by said 10th signal to produce an 1 1th signal representative thereof, means for multiplying said enlarged eighth signal and said eleventh signal to produce a 4 12th signal representative thereof,

means for taking the square root of said 12th signal to produce a 13th signal representative of a correction factor for correcting the interval during which the fuel injection valves should be held open,

means for generating an electrical signal representative of said crank angle interval, means for modifying said crank angle representative electrical signal with said 13th signal to produce a corrected signal representative of the interval during which said fuel' injection valves should -be maintained open. 1 1. Apparatus for substantially accurately measuring the air mass flow through the throat of a venturi meter used with an internal combustion engine wherein air for said engine is taken through a venturi meter comprising:

means for measuring air pressure at said venturi and producing a first signal representative thereof,

means for measuring air pressure in said idling control channel and generating a second signal representative thereof,

means for measuring ambient temperature and providing a third signal representative thereof, means for sensing barometric pressure and producing a fourth signal representative thereof,

means for determining which of said first and second signals is the larger,

means for modifying said larger signal with said third and fourth signals to provide a modified signal representative of the substantially correct air mass flow through said venturi.

' 12. A system for controlling the amount of fuel introduced into the combustion chambers or ports of an internal combustion engine by fuel injection valves fed from a common rail, said engine having a shaft driven thereby and an air intake through a venturi meter followed by an idling control channel,, the improvement comprising:

valve means for maintaining fuel pressure applied to said intake manifold from falling below a predetermined pressure value despite engine output falling below a level required for maintaining said predetermined fuel pressure value,

means for controlling said valve means for varying fuel pressure applied to said common rail above said predetermined fuel pressure value as a function of the air mass flow to said engine flowing through said venturi meter and thereafter through said idling control channel,

:electrical analog means coupled to said engine for generating a correction signal representative of errors introduced into the air mass flow function due to air compressibility and due to errors occurring when said fuel pressure is maintained at said predetermined fuel pressure value,

means for generating a crank angle signal representative of the interval required for said shaft to rotate through a predetermined angle,

means for modifying said crank angle signal with said correction signal to produce a corrected signal representative of the interval over which said fuel injection valves should remain open and means for applying said corrected signal to said fuel injection valves to hold them open over an interval represented by said corrected signal.

13. A system as recited in claim 12 wherein said means for applying said corrected signal to said fuel injection valves includes:

a source of operating potential for opening said fuel injection valves,

solenoid switch means,

a distributor,

means connecting said distributor to said fuel injection valves,

means for applying operating potential to said distributor through said solenoid switch means when said solenoid switch means is rendered operative, and

means for rendering said solenoid switch means operative over an interval represented by said corrected signal.

14. A system as recited in claim 12 wherein said means for applying said corrected signal to said fuel injection valves includes a pulse generating circuit,

a distributor connected to said pulse generating circuit, means connecting said distributor to said fuel injection valves, a discharge circuit connected to said fuel injection valves, means for establishing a time interval responsive to said corrected signal having a duration represented by said corrected signal, means for causing said pulse generating circuit to generate a pulse at the start of said time interval whereby a fuel injection valve connected to said pulse generating circuit at that time is opened and maintained open by the voltage of said pulse, and means for causing said discharge circuit to discharge the voltage of said pulse at the end of said time interval to close said fuel injection valve. 4 15. A system as recited in claim 12 wherein said electrical analog means coupled to said engine for generating a correction signal includes an adjustable attenuator for adjusting the correction signal amplitude in accordance with the engine characteristics,

a normally open switch connected across said attenuator, and means for closing said switch to short out said attenuator responsive to predetermined load requirements on said engine. 

1. A system for controlling the amount of fuel introduced into the combustion chambers or ports of an internal combustion engine by fuel injection valves fed from a common rail, each fuel injection valve being opened for fuel injection over an interval required for said engine to rotate through a fixed crank angle, said engine having an air intake with air flowing through a venturi meter, the improvement comprising: valve means for maintaining fuel pressure applied to said common rail from falling below a predetermined value despite engine output falling below a level required for maintaining said predetermined value, meAns for controlling said valve means for varying fuel pressure applied to said common rail above said predetermined pressure value as a function of the air mass flow to said engine flowing through said venturi meter, electrical analog means coupled to said internal combustion engine for generating a correction signal representative of errors introduced into the air mass flow function due to air compressibility and for errors occurring when said fuel pressure is maintained at said constant value, and means responsive to said correction signal to modify said crank angle fuel injection interval over which said injection valves are held open to compensate for said errors.
 2. A system as recited in claim 1 wherein there is included means for establishing a crank angle signal representative of the interval over which said fuel injection valves should remain open, said means for modifying the fixed crank angle interval over which said fuel injection valves are open includes means for multiplying said crank angle signal with the output of said analog means to produce a resultant signal representative of the duration for which said fuel injection valves should remain open.
 3. A system as recited in claim 1 wherein there is included an idling control channel which follows said venturi meter, and said air flowing through said venturi meter thereafter flows through said idling control channel, and said means coupled to said internal combustion engine for generating a correction signal includes means for measuring air pressure at said venturi and producing a first signal representative thereof, means for measuring air pressure in said idling control channel and generating a second signal representative thereof, means for measuring ambient temperature and providing a third signal representative thereof, means for sensing barometric pressure and producing a fourth signal representative thereof, means for determining which of said first and second signals is the larger, means for modifying said larger signal with said third and fourth signals to provide a modified signal representative of the substantially correct air mass flow through said venturi.
 4. A system as recited in claim 3 wherein there is included a means for measuring fuel pressure applied to said fuel intake manifold and establishing a fuel pressure signal representative thereof, said valve means is a spill valve having a predetermined valve area, a diaphragm is coupled to said venturi meter and to said spill valve to move said spill valve in accordance with the air pressure variations at said venturi meter, means for increasing the amplitude of said modified signal by a constant determined by the ratio of the area of said diaphragm to the area of said spill valve to produce an amplified modified signal, means for determining the ratio of said fuel pressure signal and said amplified modified signal to produce a sixth signal representative thereof, means for determining the square root of said sixth signal and producing a correction signal representative thereof.
 5. In a system as recited in claim 1 wherein there is included mode switch means for altering the amplitude of said correction signal responsive to predetermined load requirements on said internal combustion engine.
 6. In a system for controlling the amount of fuel introduced into the combustion chambers or ports of an internal combustion engine by fuel injection valves fed from a common rail, each of said fuel injection valves being opened for fuel injection over an interval determined by the time required for said engine to traverse a predetermined crank angle, said engine having an air intake through a venturi meter followed by an idlying control channel, the improvement comprising: valve means for maintaining fuel pressure applied to said common rail from falling below a predetermined value despite engine output falling below the level required to maintain said value, meAns for controlling said valve means for varying fuel pressure of fuel applied to said common rail above said predetermined pressure value as a function of the mass air flow to said engine flowing through a venturi meter and thereafter through an idling control channel, means for varying said crank angle interval over which said injection valves are held open to compensate for errors introduced into the air mass flow function because of air compressibility and for errors occurring when said fuel pressure is maintained at said predetermined value, including means for measuring air pressure at said venturi and generating a first signal representative thereof, means for measuring air pressure in the idling control channel and generating a second signal representative thereof, means for sensing fuel injection pressure to said common rail and providing a third signal representative thereof, means for measuring ambient temperature and providing a fourth signal representative thereof, means for sensing the barometric pressure and providing a fifth signal representative thereof, means for sensing the fuel temperature and producing a sixth signal representative thereof, and electrical analog means responsive to said first, second, third, fourth, fifth and sixth signals for generating a correction signal representative of an amount said fixed crank angle interval should be altered to compensate for errors introduced in the mass air flow function because of air compressibility and for errors occurring when said fuel pressure is maintained at said predetermined value caused by low engine output, and means for reducing the fixed crank angle interval over which said fuel injection valves are open responsive to said electrical analog means output.
 7. In a system as recited in claim 6 wherein there is included means for establishing a crank angle signal representative of the interval over which said fuel injection valves should remain open, said means for reducing the fixed crank angle interval over which said fuel injection valves are open includes means for multiplying said crank angle signal with the output of said analog means to produce a resultant signal representative of the duration for which said fuel injection valves should remain open.
 8. In a system as recited in claim 6 wherein there is included mode switch means for altering the amplitude of said electrical analog means output responsive to predetermined load requirements on said internal combustion engine.
 9. In a system as recited in claim 6 wherein said electrical analog means comprises means for establishing which ever of said first and second signals is the larger and producing a seventh signal representative thereof, means for combining said fifth and seventh signals for producing an eighth signal representative thereof, means for increasing said eighth signal by a predetermined gain to produce an eighth signal representative thereof, means for multiplying said seventh and sixth signals to produce a ninth signal representative thereof, means for multiplying said third and fourth signals to produce a 10th signal representative thereof, means for dividing said ninth signal by said tenth signal to produce an 11th signal representative thereof, means for multiplying said eighth signal and said 11th signal to produce a 12th signal representative thereof, and means for taking the square root of said 12th signal to produce a 13th signal representative of a correction factor for correcting the interval during which the fuel injection valve should be held open.
 10. In a system for controlling the amount of fuel introduced into the combustion chambers or ports of an internal combustion engine by fuel injection valves fed from a fuel intake manifold, each fuel injection valve being opened for fuel injection over a crank angle interval required for the engine to rotate through a predetermined crAnk angle, and the pressure above a predetermined threshold at which said fuel is applied to each said injection valve is varied as a function of the air mass flow to said engine through a venturi meter and thereafter through an idling control channel, means for correcting the interval over which said fuel injection valve remains open to compensate for errors in fuel pressure which occur due to errors introduced by the use of a venturi meter for measuring air mass flow to said engine, and to control the richness of the fuel mixture when said fuel pressure attains its predetermined threshold value comprising: first means for sensing the pressure of air at said venturi throat and establishing a first signal representative thereof, second means for sensing the air pressure in the idling control channel which is downstream of said venturi throat and producing a second signal representative thereof, third means for sensing fuel pressure applied to a fuel intake manifold and producing a third signal representative thereof, fourth means for sensing ambient temperature and producing an electrical signal representative thereof, fifth means for sensing the barometric pressure and producing a fifth signal representative thereof, sixth means for sensing the fuel temperature and producing a sixth signal representative thereof, means for establishing whichever is the larger of said first and second signals and producing a seventh signal representative thereof, means for combining said fifth and seventh signals for producing an eighth signal representative thereof, means for increasing said eighth signal by a predetermined gain to produce an enlarged eighth signal representative thereof, means for multiplying said seventh and sixth signals to produce a ninth signal representative thereof, means for multiplying said third and fourth signals to produce a 10th signal representative thereof, means for dividing said ninth signal by said 10th signal to produce an 11th signal representative thereof, means for multiplying said enlarged eighth signal and said eleventh signal to produce a 12th signal representative thereof, means for taking the square root of said 12th signal to produce a 13th signal representative of a correction factor for correcting the interval during which the fuel injection valves should be held open, means for generating an electrical signal representative of said crank angle interval, means for modifying said crank angle representative electrical signal with said 13th signal to produce a corrected signal representative of the interval during which said fuel injection valves should be maintained open.
 11. Apparatus for substantially accurately measuring the air mass flow through the throat of a venturi meter used with an internal combustion engine wherein air for said engine is taken through a venturi meter comprising: means for measuring air pressure at said venturi and producing a first signal representative thereof, means for measuring air pressure in said idling control channel and generating a second signal representative thereof, means for measuring ambient temperature and providing a third signal representative thereof, means for sensing barometric pressure and producing a fourth signal representative thereof, means for determining which of said first and second signals is the larger, means for modifying said larger signal with said third and fourth signals to provide a modified signal representative of the substantially correct air mass flow through said venturi.
 12. A system for controlling the amount of fuel introduced into the combustion chambers or ports of an internal combustion engine by fuel injection valves fed from a common rail, said engine having a shaft driven thereby and an air intake through a venturi meter followed by an idling control channel,, the improvement comprising: valve means for mainTaining fuel pressure applied to said intake manifold from falling below a predetermined pressure value despite engine output falling below a level required for maintaining said predetermined fuel pressure value, means for controlling said valve means for varying fuel pressure applied to said common rail above said predetermined fuel pressure value as a function of the air mass flow to said engine flowing through said venturi meter and thereafter through said idling control channel, electrical analog means coupled to said engine for generating a correction signal representative of errors introduced into the air mass flow function due to air compressibility and due to errors occurring when said fuel pressure is maintained at said predetermined fuel pressure value, means for generating a crank angle signal representative of the interval required for said shaft to rotate through a predetermined angle, means for modifying said crank angle signal with said correction signal to produce a corrected signal representative of the interval over which said fuel injection valves should remain open and means for applying said corrected signal to said fuel injection valves to hold them open over an interval represented by said corrected signal.
 13. A system as recited in claim 12 wherein said means for applying said corrected signal to said fuel injection valves includes: a source of operating potential for opening said fuel injection valves, solenoid switch means, a distributor, means connecting said distributor to said fuel injection valves, means for applying operating potential to said distributor through said solenoid switch means when said solenoid switch means is rendered operative, and means for rendering said solenoid switch means operative over an interval represented by said corrected signal.
 14. A system as recited in claim 12 wherein said means for applying said corrected signal to said fuel injection valves includes a pulse generating circuit, a distributor connected to said pulse generating circuit, means connecting said distributor to said fuel injection valves, a discharge circuit connected to said fuel injection valves, means for establishing a time interval responsive to said corrected signal having a duration represented by said corrected signal, means for causing said pulse generating circuit to generate a pulse at the start of said time interval whereby a fuel injection valve connected to said pulse generating circuit at that time is opened and maintained open by the voltage of said pulse, and means for causing said discharge circuit to discharge the voltage of said pulse at the end of said time interval to close said fuel injection valve.
 15. A system as recited in claim 12 wherein said electrical analog means coupled to said engine for generating a correction signal includes an adjustable attenuator for adjusting the correction signal amplitude in accordance with the engine characteristics, a normally open switch connected across said attenuator, and means for closing said switch to short out said attenuator responsive to predetermined load requirements on said engine. 